It has been proposed to use hydrogen as a “clean” fuel for vehicles. Such hydrogen-powered vehicles may convert the hydrogen fuel into energy using fuel cells or may use the hydrogen fuel in an internal combustion engine.
Hydrogen fuel may be stored on board the vehicle in the form of high pressure gas (typically 20 to 80 MPa maximum working pressure). However, one serious problem involving the use of high pressure gaseous hydrogen fuel is the risk of a crash causing a rupture of the high pressure hydrogen storage vessel which might result in a sudden explosive pressure wave followed by an immediate deflagration fire.
The alternative to storing hydrogen on board the vehicle in the form of a high pressure gas is to store it in liquid form at low pressure in a cryogenic storage tank. The consequence of rupturing a tank of liquid hydrogen would be the release of hydrogen at low pressure. Such release would result in a fire hazard rather than an explosion risk. Studies have shown that a liquid hydrogen fire is likely to have less severe consequences than a gasoline fire. There are, therefore, clear advantages derivable from storing hydrogen fuel for road vehicles in liquid form rather than as a high pressure gas.
There are two possible methods of delivering hydrogen to fuel outlets. One method involves delivering the hydrogen as a gas in pipelines or in high pressure cylinders transported on road or rail vehicles. The second method involves delivering hydrogen as a liquid carried in insulated tankers. Current technology and road regulations (44 metric tons all-up weight limit in Europe) would allow a payload of about 600 kg of hydrogen as a gas or about 4000 kg of liquid hydrogen. These limits make delivery of hydrogen gas by road a very expensive method compared to the delivery of liquid hydrogen. In this connection, a recent report by the US National Renewable Energy Authority (“Hydrogen Supply: Cost Estimate For Hydrogen Pathways—Scoping Analysis”; D. R. Simbeck and E. Chang, Jul. 2002) gives the total cost (including production, delivery and dispensing) of liquid hydrogen distribution as $3.66/kg compared with $4.39/kg for gaseous hydrogen distribution. These figures are based on using a refrigeration power of 11 kWh/kg. There is, therefore, a clear cost advantage for liquid hydrogen production and delivery when compared with that for gaseous hydrogen. The delivered cost of hydrogen in road vehicles is highly dependent on the delivery distances involved, with the delivered cost of gaseous hydrogen increasing much faster than that for liquid hydrogen.
Projected performance of fuel cell powered cars would indicate that the 4000 kg liquid hydrogen payload would provide approximately the same mileage as the 25 metric ton petrol payload for the current largest petrol tankers used for fuel delivery. Such an amount of liquid hydrogen corresponds with about 200,000 miles of driving for 25 metric tons of petrol assuming an average consumption of about 30 mpg. Various figures have been published for the likely hydrogen consumption of fuel-cell powered vehicles such as cars, SUVs and vans which range from an equivalent energy content of from 50 to 100 mpg. Taking an average of 75 mpg, if liquid hydrogen were used for providing hydrogen fuel to outlets, then it would require about 3.6 metric tons of liquid hydrogen to provide 200,000 miles of driving and the number and type of fuel delivery vehicles would be virtually unchanged compared to current petrol and diesel based fuel systems.
Liquid hydrogen can be raised to a high pressure, e.g. 30 to 80 MPa, using a single stage reciprocating pump followed by heating the pressurized liquid to ambient temperature. This process has lower capital and operating costs that a corresponding hydrogen gas compression system. Therefore, it is also more cost effective to provide pressurized liquid hydrogen as a source of ultra high pressure hydrogen gas at a refueling station for vehicles running on pressurized hydrogen gas.
Large scale production of liquid hydrogen was first carried out in the 1950s as a consequence of the demand for liquid hydrogen as rocket fuel. However, it was quickly apparent that liquid hydrogen could not be used economically as a fuel for road vehicles due to the high power consumption required during hydrogen liquefaction which results in prohibitively high production costs.
The technology of liquid hydrogen production has been under continuous development since the 1950s based on the objective of lower specific power consumption and lower capital costs. For example, current liquefiers have centrifugal expanders and plate fin heat exchangers in place of reciprocating expansion engines and wound coil tubular heat exchangers. In addition, lower pressure hydrogen liquefaction cycles have been developed, together with new ortho-para conversion catalysts. Improvements have been observed by depositing ortho-para conversion catalysts in the feed passages of the heat exchanger and by optimizing the mechanical design layout, control, instrumentation and insulation systems.
Early hydrogen liquefier cycles used hydrogen as the working fluid although, more recently, hydrogen has been replaced with a mixture of gases, for example a mixture of neon and helium, to increase the molecular weight and allow the use of fewer stages of compression and expansion than are required if hydrogen is used as the working fluid. It has also been suggested that evaporating propane or other suitable refrigerants should be used for the first stages of the hydrogen feed cooling.
A hydrogen liquefaction process typically involves two stages; an initial pre-cooling stage and a subsequent liquefaction stage. Hydrogen must be cooled below its upper Joule-Thomson inversion temperature, i.e. the point below which a gas cools when expanded, before it can be liquefied in a recuperative liquefaction cycle. Generally, this means that hydrogen must be cooled below about −150° C. in a pre-cooling step before entering the liquefaction stage of the process.
It is well appreciated in the art that hydrogen gas exists in about 75% ortho form and about 25% para form and that, if hydrogen gas is liquefied without conversion of the ortho form to the para form, then the resultant liquid hydrogen evaporates more easily than liquid hydrogen in predominately the para form. The ortho form of hydrogen gas is, therefore, usually converted catalytically to the para form prior to liquefaction.
In certain known hydrogen liquefaction processes, hydrogen gas is pre-cooled to below about −150° C. by indirect heat exchange against vaporizing liquid nitrogen and/or cold nitrogen gas streams. An example of such a known process is disclosed in U.S. Pat. No. 3,109,725 (Flynn) in which hydrogen gas is pre-cooled by indirect heat exchange against liquid nitrogen in which hydrogen gas is dissolved. A further example is disclosed in U.S. Pat. No. 4,765,813 (Gaumer, Jr. et al) in which hydrogen gas is pre-cooled using liquid nitrogen and then further cooled using a closed neon refrigeration cycle. Compression duty in the refrigeration cycle takes place at the warm end of the cycle.
U.S. Pat. No. 2,983,585 (Smith) discloses a partial oxidation process in which methane is partially oxidized with oxygen to produce carbon monoxide and hydrogen gas. The partial oxidation process is integrated with a hydrogen liquefaction process in which hydrogen gas is pre-cooled by indirect heat exchange against liquid methane and subsequently further cooled against a closed external refrigerating cycle using liquid nitrogen (“LIN”) as the refrigerant. The resultant methane is compressed at the warm end of the liquefaction process and then fed to the partial oxidation process. The resultant gaseous nitrogen is compressed at the warm end of the closed cycle before being condensed by indirect heat exchange with liquid methane and recycled. It is disclosed that the liquid methane could be replaced with liquefied natural gas (“LNG”).
U.S. Pat. No. 3,347,055 (Blanchard et al) discloses a process in which a gaseous hydrocarbon feedstock is reacted to produce hydrogen gas which is then liquefied in an integrated liquefaction cycle. In one embodiment, the liquefaction cycle involves two closed refrigerant cycles, the first using hydrogen gas a refrigerant and the second using nitrogen. Compression for both refrigeration cycles takes place at the warm end of the cycles. The hydrogen to be liquefied is also cooled by indirect heat exchange against liquefied hydrocarbon feedstock gas thereby producing gaseous feedstock at 1 atm. (e.g. about 0.1 MPa) for use in the hydrogen production plant. It is disclosed that the hydrocarbon feedstock may be natural gas.
JP-A-2002/243360 discloses a process for producing liquid hydrogen in which hydrogen feed gas is pre-cooled by indirect heat exchange against a stream of pressurized LNG. The pre-cooled hydrogen gas is fed to a liquefier where it is further cooled by indirect heat exchange against both LIN and a refrigerant selected from hydrogen or helium. The further cooled hydrogen is then expanded to produce partially condensed hydrogen which is separated into liquid hydrogen, which is removed and stored, and hydrogen vapor which is recycled around the liquefier.
Quack discloses (“Conceptual Design of a High Efficiency Large Capacity Hydrogen Liquefier”; Adv. Cryog. Eng., Proc. CEC, Madison 2001, AIP, Vol. 613, 255-263) a hydrogen liquefier cycle that, to the inventors knowledge, most accurately represents the best current technology projections for hydrogen liquefaction cycles. It should be noted that Quack uses efficiency figures for compressors and turbines that are not achievable at present but which are thought to be realistic for the future.
In the hydrogen liquefaction cycle proposed by Quack, a two stage pre-cooling process is employed. Hydrogen gas at 300K (about 27° C.) and at a pressure of about 8 MPa is cooled to 220K (about −53° C.) by indirect heat exchange against propane as refrigerant. Quack suggests that other refrigerants such as ammonia, fluorocarbons or mixtures of different refrigerants could also be used for this step. The hydrogen gas at 220K (about −53° C.) is then further cooled to about 73K (about −200° C.) by indirect heat exchange against a helium/neon mixture as refrigerant. It is suggested that this vapor compression refrigeration step may use a mixture of refrigerants or gas cycles with nitrogen, hydrogen or helium as refrigerant. The hydrogen gas at 73K (about −200° C.) is then further cooled to 25K (about −248° C.) by indirect heat exchange against a mixture of helium and neon. The further cooled hydrogen gas at 25K (about −248° C.) is then work expanded to partially liquefy the hydrogen. The cycle uses ortho-para conversion catalyst optimally arranged at the cold end of the plant and assumes the usual optimal placement of heat exchangers and turbines in the hydrogen circuit at the cold end of the plant.
Current hydrogen liquefaction processes consume power at a rate of about 11 kWh/kg(liquid hydrogen) based on a gaseous hydrogen feed at a typical pressure of 2.5 MPa (25 bar). Quack suggests that the best future power consumption will be in the range 5 to 7 kWh/kg(liquid hydrogen) if his suggested improvements are utilised.
One of the major features of the Quack cycle is that all of the main hydrogen compression, propane recycle compression and helium/neon recycle compression takes place with each compressor stage operating at close to ambient temperature using ambient temperature cooling systems to remove heat of compression such as cooling water or air cooling. There is, however, a small cold flash-gas compressor operating at the cold end of the heat exchanger.
Compressors operating at the warm end of a cryogenic cycle are usually cooled using water or air as a coolant.
It is an objective of preferred embodiments of the present invention to reduce power consumption in hydrogen liquefaction processes to such an extent that liquid hydrogen as a vehicle fuel becomes not only a viable economic alternative to gasoline but also the natural choice over pressurized hydrogen gas as the form in which hydrogen is delivered to vehicle refueling stations either by road vehicle or by pipeline.
The low molecular weight of hydrogen means that a very large number of stages of compression must be used if centrifugal compressors are specified and, likewise for a given expansion duty, a large number of centrifugal expansion stages will be required operating in series. In current plants, the hydrogen compressors are often reciprocating units. Multiple stage compressors plus large recuperative heat exchangers will usually be used in the pre-cooling steps of the process.
It is, therefore, also an objective of preferred embodiments of the present invention to reduce the capital cost of hydrogen liquefaction, in particular, by targeting the pre-cooling stage of the process.
LNG is produced and stored in vast quantities in numerous locations around the world. Such storage facilities pressurize and heat LNG before supplying the resultant pressurized natural gas to pipelines for distribution to industry and homes. The inventors have realized that, by using the internal energy of pressurized LNG to provide part of the energy required to liquefy hydrogen, it is possible to efficiently convert this vast source of energy to effective power which appears as a power reduction in a hydrogen liquefaction process. In effect, pressurized LNG absorbs heat at low temperature liberated from a hydrogen liquefaction plant.
At present, LNG storage and distribution facilities pump LNG to high pressure, usually between from about 3 to about 10 MPa, and then heat the pressurized LNG using natural gas burners submerged in a water bath. The burners use a small portion, e.g. 1 to 2%, of the ambient temperature natural gas as a fuel to heat the remaining pressurized LNG to ambient temperature.
It is, therefore, a further objective of preferred embodiments of the present invention to reduce the quantity of pressurized LNG needed to provide the heat required to heat pressurized LNG thereby producing high pressure natural gas for supply to pipelines.